1. Technical Field
The invention relates generally to apparatus for calendering a traveling web, such as a web of paper, and more particularly to apparatus and methods for supporting multiple calender rolls on a paper making machine.
2. Related Prior Art
It is generally known to treat a travelling web, such as a web of paper or paper board, by means of calendering to provide desirable surface properties. For example, the smoothness and degree of gloss provided by the web's surfaces can be varied by calendering.
Many grades of paper and paper board need to be calendered on both sides of the web so as to achieve the desired high quality finishes of those surfaces. To provide such finishing, it is generally known to pass the web through a first nip provided by a first calender assembly to treat one side of the web and through a second nip provided by a second calender assembly to treat the second side of the web. Moreover, many grades of paper need multiple passes through calender nips on each side of the web to achieve the desired finish characteristics. To achieve such finishing, it is known to provide multiple calendering apparatus arranged in either a vertical configuration or a horizontal configuration.
A vertical calender provides a pair of calender rolls mounted for rotation about respective axes which lie in a substantially vertical plane. In general, such vertical calenders provide finishing apparatus having a relatively high machine efficiency and better control of finish on the web. Machine efficiency is improved because the web follows a relatively horizontal straight path through the nip, thus making threading of the web along the machine easier and facilitating control of tension on the web. Also, machine efficiency is enhanced by the relatively short amount of space in the machine direction required for a single pair of vertically arranged calender rolls. In addition, should the web break, vertical calender arrangements are somewhat easier to clear of broke.
In applications wherein multiple calender passes are necessary, passing a web through a series of vertical calenders to alternatingly treat the opposite sides of the web increases control over the finish on the opposed sides of the web. Also, the rolls in each successive calender can be individually controlled to provide the appropriate web surface treatment.
A horizontal calender provides a pair of calendering rolls mounted for rotation on respective axes lying in a generally horizontal plane. Such a horizontal configuration often includes a third roller mounted for rotation in the common horizontal plane such that the web travels vertically between a first nip formed by the first and second rolls and subsequently through a second nip formed by the second and third rolls. In soft calendering, such a horizontal arrangement provides two nips which treat a single side of the web. Thus, for providing two nips, a horizontal calender entails a relatively low capital cost and requires less space in the machine direction when compared to a vertical single nip arrangement. The capital cost is relatively low for a three-roll horizontal calender because two nips can be formed using only three rolls compared to four rolls if vertical calenders are used. Also, a horizontal calender provides a finish treating apparatus that is somewhat easier to control than a vertical calender because the nip pressure applied to the web can be controlled by adjusting the spacing between the calender rolls through horizontal movements that need not overcome the weight of the calender rolls, i.e., the calender rolls need not be lifted into or out of engagement with the web. As a result, nip pressure adjusting apparatus associated with a horizontal calender need not be as powerful as the apparatus needed to adjust a comparable vertical calender assembly.